This invention relates to the application of coatings onto substrates by plasma spray techniques, and more particularly, to method and apparatus for shielding the effluent from plasma spray gun assemblies from contamination by the surrounding environment.
Plasma spray gun assemblies are known which use an electric arc to excite a gas, thereby producing a thermal plasma or very high temperature. Spray or powdered materials are introduced into the thermal plasma, melted and projected onto a substrate or base to form coatings. Such powdered materials may include metals, metal alloys, ceramics such as metal oxides, and carbides or the like, for example.
Heretofore, difficulties were experienced due to contamination of the effluent from the nozzle of the spray gun, such as air entrapment, for example, that resulted in significant oxidation of the coating materials. The spraying conditions, particularly heat and velocity, were often adjusted to a compromise to heat the powder just enough to melt it. Attempts have been made to overcome this problem, but they have been only moderately successful. One such attempt involved completely enclosing the apparatus in a chamber, but this was expensive and also very cumbersome. In other installations, efforts were made to use a gas shroud to solve the problem. For example, the Jackson U.S. Pat. No. 3,470,347 shows the use of a coaxial annular stream of unheated gas. However, this required a relatively large flow of gas, such as argon, which is expensive. In addition, there was a tendancy with such prior art devices to build up a coating on the shrouding device. Other related patents in this art include Anderson et al, U.S. Pat. No. 2,951,143; Yoshiaki Arata et al, U.S. Pat. No. 3,082,314; and Unger et al, U.S. Pat. No. 3,313,909, for example.